Display device

ABSTRACT

The invention relates to a display device for displaying an image comprising a plurality of display pixels ( 2 ), a controller ( 3 ) for generating a driving signal ( 8 ) for driving the pixels ( 2 ), and sensors ( 9; 11; 14 ), wherein the sensors ( 9; 11; 14 ) are able to monitor operating conditions of the pixels ( 2 ), and the controller ( 3 ) is adapted to receive data related to the operating conditions from the sensors ( 9; 11; 14 ) to determine a brightness change of the pixels ( 2 ) caused by the operating conditions and to generate the driving signal ( 8 ) in dependence on the brightness change.

The invention relates to a display device for displaying an imagecomprising a plurality of display pixels, a controller for generating adriving signal for driving the pixels, and sensors. The invention alsorelates to a method of generating a driving signal for driving aplurality of pixels of an organic electroluminescent display device fordisplaying an image.

The display pixels in organic electroluminescent display devices such aspoly-or or organic light emitting diode (PLED and OLED respectively)display devices, hereinafter referred to as display devices, degradeduring operation, resulting in a change (usually a reduction) in lightoutput at a given current density. An example of such degradationbehavior is illustrated in FIG. 1, showing a reduction of light output Las a function of the operation time t. The display is driven at aconstant current. As the light output L decreases, the driving voltage Dincreases. As some pixels in a display are used more often than otherpixels, these more frequently used pixels exhibit a larger degradationthan pixels that have been used less frequently. This phenomenon resultsin a burnt-in image in the display device. In color displays, theeffects are even more serious as the display suffers from discoloration,i.e. “white” is no longer white, but exhibits e.g. a green shade.

WO 99/41732 discloses a tiled electronic display structure wherein eachtile comprises an integrated circuit connected to the various displaypixels of that tile. The integrated circuit includes an electroniccompensation system which continuously adjusts the brightness of theindividual display pixels to compensate for aging or degradation. Theelectronic compensation is achieved by predicting the decay in thebrightness of the display pixel by measuring the current and time forthat particular pixel and integrating the product of current and time,i.e. the total charge data This product is fitted to a characteristiccurve and used to adjust the drive current by predicting a new drivecurrent which restores the original brightness level of the pixel.

However, monitoring the total charge data for a display pixel in manyinstances is insufficient to reliably establish the requiredcompensation for restoring the original brightness level of the displaypixel or to maintain a uniform brightness.

It is an object of the invention to provide an improved display devicethat is able to maintain a more uniform brightness level of the displaypixels. The invention is defined by the independent claims. Thedependent claims define advantageous embodiments.

This object is achieved by providing a display device wherein thesensors are able to monitor operating conditions of the pixels and thecontroller is adapted to receive data related to the operatingconditions from the sensors to determine a brightness change of thepixels caused by the operating conditions and to generate the drivingsignal in dependence on the brightness change.

By providing such a display device the data from the sensors (or sensor)are available in order to generate a driving signal for the displaypixels that sufficiently takes into account relevant factorscontributing to the degradation of the display pixels. This enables amore accurate determination of the brightness change of the pixels thanthe prior art.

It is advantageous if the controller is able to provide the pixels witha substantially constant relative brightness when the image isdisplayed. The relative brightness of the pixels could be used to adjustthe driving signal to the level of the pixel with the worst degradationby reducing the drive of the less degraded pixels. This extends thelifetime of the pixels. Pixels with a value beyond a predetermined levelof degradation could be excluded from the selection of the worstdegraded pixel. Alternatively the relative brightness can be used toadapt the driving signal to restore the initial brightness level or torestore a level in-between the initial level and the level of the worstdegraded pixel.

In a preferred embodiment of the invention the sensors comprise at leastone temperature sensor for monitoring temperature data relating to thepixels; monitoring means are present for monitoring total charge data ofthe pixels and the controller is adapted to generate the driving signalin dependence on the total charge data and the temperature data. Thisembodiment enables adjustment of the driving signal if the operatingtemperature varies as a result of which the degradation behavior of thedisplay pixels changes. The temperature data may be expressed as anacceleration factor that is used as a multiplier for the total charge toobtain the improved driving signal to provide the pixels with asubstantially constant relative brightness.

It is advantageous if the temperature sensor comprises at least onereference pixel and temperature determination means adapted to determinea temperature in dependence on at least one temperature-dependentcharacteristic of the reference pixel. The reference pixel used formeasuring or deriving the temperature is manufactured at the displaydevice simultaneously with the display pixels, so no additional processsteps have to be carried out for providing the temperature sensor.Moreover, the temperature of the display or display pixels may bemeasured or derived more reliably than when a different construction ofa sensor is applied, since the reference pixel(s) used for temperaturesensing is(are) an integral part of the display device, so a directmeasurement can be performed. The temperature-dependent characteristicor value may relate to an electrical characteristic or value, such asthe conductivity of the reference pixel.

Preferably, the material composition of the reference pixel is similarto that of the display pixel, since this is advantageous with regard todecreased complexity of the manufacturing process of the display device.

In a preferred embodiment of the invention, the reference pixel isdriven in accordance with a temperature measurement state. In thetemperature measurement state the reference pixel is biased at a levellow enough to prevent, or at least substantially prevent, the pixel fromemitting light and high enough to enable a reliable measurement orderivation of the temperature-dependent characteristic or value of thereference pixel. Biasing the reference pixels in accordance with thetemperature measurement state has the advantage that the pixels do notexhibit degradation behavior that is usually observed when pixels aredriven to emit light. Therefore, measurement of the temperature can beperformed reliably and no correction is needed to account for thedegradation of the reference pixel. In biasing the reference pixels,both reverse and forward bias may be employed. The reference pixel canalso be regularly probed, depending e.g. on the correction drivingscheme applied. Regular probing may be more efficient with regard topower consumption as compared to a continuous measurement.

In a preferred embodiment of the invention, the reference pixel isshielded from ambient or environmental light. Shielding of the referencepixel(s) from ambient light prevents photocurrents from influencing themeasurement and prevents possible degradation of the reference pixel(s)due to ambient light.

In a preferred embodiment of the invention, the sensors comprise atleast one reference pixel, e.g. a dummy pixel, monitoring means arepresent for monitoring total charge data of the pixels and furthermonitoring means are present, adapted for determining degradation statedata of the reference pixel, the controller being adapted to generatethe driving signal taking account of the total charge data and thedegradation state data The incorporation of one or more reference pixelsin the display device enables adjustment of the driving signal takinginto account other effects, such as spontaneous degradation of displaypixels (shelf life effect) and deviations from the expected degradationbehavior of the display pixel especially occurring at the beginning ofthe life time of the display device (initial drop effect). Preferably areference pixel has an associated photodiode for directly measuring thedegradation state or for deriving a degradation state of the referencepixel.

In a preferred embodiment of the invention, the driving signal takesinto account the total charge data from the monitoring means and thetemperature data and the degradation state data from the furthermonitoring means. This enables the device to more reliably monitor thedegradation of the display pixels and to generate an improved drivingsignal to restore the original brightness level of the display pixel.

In a preferred embodiment of the invention, the display is a colordisplay wherein the pixels comprise at least two sub-pixels of adifferent type and at least one reference pixel for each type ispresent. Advantages of this embodiment reside in that degradationbehavior of, for example, R, G and B display sub-pixels of a differenttype could differ significantly from each other as a consequence ofwhich the adjustment of the driving signal is different for the R, G andB sub-pixels. Moreover, this embodiment enables the display device tomaintain the required color balance. Furthermore, active matrix colordisplays can be monitored easily in this way since the voltage acrossthe pixels in the array does not have to be measured anymore in order toobtain total charge data of these pixels. If dummy pixels are applied,each different type preferably is represented by a minimum of one dummypixel. It is noted that if color displays are discussed in thisapplication, the term “display pixel” also refers to each of theindividual R, G and B sub-pixels.

In a preferred embodiment of the invention, the dummy pixels are drivenat an average brightness level for each color. This embodimenteliminates the need to pre-age the displays prior to customer deliveryand thereby decreases the manufacturing costs.

In a preferred embodiment of the invention, it is possible to turn offthe adjustment of the driving signal due to data received from themonitoring means and/or further monitoring means for one or more colordisplay pixels. This provides an advantage in that if serious deviationsfrom the expected degradation behavior are encountered, extremeover-compensation, that may lead to early failure of the display, can beavoided.

It will be appreciated that the previous embodiments or aspects of theprevious embodiments of the invention can be combined.

In the embodiments described above the data or derivatives thereof arestored for preferably each individual display pixel. As a furtherembodiment the sensors comprise circuitry to sense a relation between areverse current and a reverse voltage of the pixels for derivingdegradation state data for the pixels, and the controller is adapted togenerate the driving signal taking account of the degradation statedata. This embodiment has the advantage that storage of a pixel historyin a memory is no longer required, since the actual degradation state ofa pixel is derived from sensing the relationship between reverse voltageand reverse current. The applied reverse current or reverse voltage ispreferably chosen in accordance with the size of the display pixel.

In a preferred embodiment, the degradation state data are derived afterturning on the display device. In this way an absolute determination ofdegradation state is available at each turn on. This may be especiallyimportant if the required adjustment of the driving signal is not linearin time.

The embodiments of the invention will be described in more detail belowwith reference to the attached drawings, in which:

FIG. 1 schematically shows a typical degradation behavior of a LEDdevice driven at a constant current;

FIG. 2 shows a LED display device according to a first embodiment of theinvention;

FIG. 3 shows a LED display device according to a second embodiment ofthe invention;

FIG. 4 is a schematic representation of the brightness decay as afunction of the fractional lifetime for two types of LED displaydevices;

FIG. 5 shows a LED display device according to an alternative embodimentof the invention;

FIG. 6 shows a measurement result of the leakage current as a functionof an applied reverse voltage at different lifetimes of a LED displaydevice;

FIG. 7 shows the shift in the normalized reverse voltage as a functionof the lifetime of the LED display device at different leakage currents;

FIG. 8 shows a LED display device according to an embodiment of theinvention; and

FIG. 9 shows a schematic representation of a typical current/voltagecharacteristic of a PLED device.

FIG. 2 shows a display device according to a preferred embodiment of thefirst invention wherein means are provided to compensate for thedegradation behavior of a LED device as depicted in FIG. 1. A display Icomprises a plurality of display pixels 2 arranged in a matrix of rowsand columns. The display pixels 2 can be driven by a controller 3 inresponse to a data input signal 4. The data input signal 4 comprisese.g. one or more images to be shown on the display 1 by driving theindividual display pixels 2. It will be appreciated that the display 1may be a passive or an active matrix display and may be either amonochrome display or a color display in which the display pixelscomprise sub-pixels, for example, R, G and B.

At a given temperature the rate of light degradation for the pixels 2 inthe display 1 scales fairly linearly with the current density in thedevice, whilst the overall degradation rate decreases (oftenlogarithmically) as the device is used more (FIG. 1). For color displaysusing different types of light emitting materials for each color, e.g.R. G and B, the absolute rate of decay of the light output varies forthe different sub-pixels R, G and B.

In order to monitor this degradation, the display 1 incorporates amodule 5 connected to the controller 3 by a connection 6. The module 5is adapted to monitor the total charge which has passed through a pixel2 at a given time, i.e. the pixel history. It is to be noted that themodule 5 may be an integral part of the controller 3, but will be drawnseparately for reasons of clarity. The module 5 comprises a look-uptable (not shown) and/or an analytical function and is suited to providethe controller 3 via a connection 7 with data concerning the degradationof the display pixels. In color displays a separate look-up table oranalytical function can be used for each sub-pixel. The controller 3drives the display pixels 2 by generating a driving signal 8 that may beadjusted to compensate for the monitored degradation of one or moredisplay pixels 2.

During operation, the controller 3 receives a data input signal 4 to bedisplayed on the display 1 by driving the display pixels 2. Datamanipulation may be performed by or in the controller 3 or by or in themodule 5. If a fall image of data is to be adjusted simultaneously, thedata may be stored locally in the controller 3 in a simple frame memory.Alternatively, if smaller portions of image data are to be modified, acorrespondingly smaller memory may be sufficient, such as a line memory.In module 5, the pixel history of a display pixel 2 is accessed andtransferred to controller 3 via connection 7. In controller 3, with thehelp of the look-up table or analytical function, the data input signal4 temporarily stored in the local memory of the controller 3 is adjustedto data signal 4′ (not shown) to account for the pixel history. Theadjusted data signal 4′ is transferred to module 5 via connection 6 andis added to the previous pixel history and stored in module 5 as the newpixel history. Data signal 4′ is also used as the adjusted drivingsignal 8 for driving the display pixels 2 so as to maintain the relativebrightness level of the pixels 2.

Alternatively, the data 4 of the input signal 4 to be displayed on thepixels 2 is directly transferred to module 5 via connection 6. If a fullimage of data is to be adjusted simultaneously, the data may be storedlocally in module 5 in a simple frame memory. If smaller portions ofimage data are to be modified, a correspondingly smaller memory will besufficient. In module 5, the pixel history of the pixels 2 is accessedand with the help of the look-up table or analytical function the datainput signal 4 is adjusted to data signal 4′ to account for the pixelhistory. The adjusted data signal 4′ is added to the previous pixelhistory and stored in module 5 as the new pixel history. Data signal 4′is also transferred to module 3 using connection 7 in order to obtainthe adjusted driving signal 8 for driving the display pixels 2 so as tomaintain the same relative brightness level. Alternatively, thebrightness of less degraded display pixels can be reduced by adjustingthe driving signal 8 to that of the most degraded display pixels 2 inorder to prolong the display lifetime.

For color display sub-pixels not only the degradation of each pixelneeds to be monitored, but also the color balance must be maintained byadjusting the driving signal, i.e. brightening (or dimming) sub-pixelsof different colors in such a manner that the color balance ismaintained. This adjustment can be done with regard to the brightness ofa non-degraded pixel or the most degraded display pixel, or, accordingto an alternative scheme, with regard to, for example, a level inbetween the two mentioned brightness levels.

Often the degradation rate of the display pixels 2 decreases as thepixels become older, as shown in FIG. 1. Therefore, progressively fewerdata from the monitoring means might be stored in the memory of themodule 5 while maintaining a certain level of precision.

The display device described so far may maintain a sufficiently stablebrightness if the device operates in a very small temperature range orthe degradation of the display pixels 2 is not stronglytemperature-dependent. However, in many instances LEDs degrade faster athigher temperatures. For obtaining reliable degradation data for thedisplay pixels 2, it can be essential to take the operating temperatureof the display pixels 2 into account. In order to obtain data concerningthe operating temperature of the display pixels 2, the display deviceincorporates at least one temperature sensor 9. For larger displays 1more temperature sensors 9 may be required to account for temperaturegradients across the display 1. The temperature sensors 9 are connectedto the controller 3 by connections 10.

In operation, the temperature of the display pixels 2 is monitored bythe temperature sensors 9 and the temperature data are fed throughconnections 10 to the controller 3. The temperature data are used todetermine an acceleration factor which may be different for each type ofcolor sub-pixels, for example, R, G, B in a color display device. Theacceleration factor reflects the different rate of degradation at eachtemperature, which degradation rate is known (for each color). The dataare adjusted as described previously, again by using e.g. look-up tablesor analytical functions in the module 5. The look-up tables oranalytical functions may be modified for the operating temperatureobtained from the temperature sensors 9. This ensures atemperature-independent display brightness and maintenance of a propercolor balance in color displays. After calculation of the associatedfall in light efficiency, the adjusted data signal 4′ is sent to thecontroller 3 via connection 7 and the drive signal 8 is adjusted tomaintain the relative brightness level of the display pixels 2. Thedrive signal 8 thus is adjusted by taking into account the drive signalpixel history (by monitoring the total charge data). The pixel historyis updated by adding the product of the adjusted data signal 4′ and thetemperature-dependent degradation acceleration factor to the previouspixel history to be stored in module 5 as a new pixel history. In colordisplays the color balance may again be maintained as describedpreviously.

In the previous embodiment as shown in FIG. 2, it was assumed that thepixel history and the temperature history of the (colored) displaypixels 2 is completely reproducible. However, several situations mightbe encountered wherein this assumption is not valid. It is e.g. knownfrom experience that display pixels 2 might also degrade without beingdriven by a driving signal 8. This effect will hereinafter be referredto as the shelf life effect. Moreover, there are periods in thedegradation, especially at the start of the lifetime of the display 1,where the degradation occurs rapidly and in a less well-defined manner,hereinafter referred to as the initial drop effect.

To account for the shelf life effect, the initial drop effect and othereffects, in FIG. 3 a display device is shown that incorporates referencepixels 11, hereinafter also referred to as “dummy” pixels. Referencenumerals identical to those used in FIG. 2 indicate the same or similarelements. The number of dummy pixels 11 is preferably small, with aminimum of one dummy pixel 11 for each different type of sub-pixel if acolor display device is used. In case two different colors are generatedwith the same type of sub-pixels in combination with different colorfilters for the sub-pixels, then for those two sub-pixels of the sametype only one common reference pixel could be used. Connections 12 areused to attach the dummy pixels 11 via further monitoring units 13, suchas light, voltage or current measurement facilities to the controller 3so as to monitor either light output, voltage (at a given current) orcurrent (at a given voltage) of the dummy pixels 11. The lightmeasurement X could be facilitated by providing each of the dummy pixelswith an associated photodiode (not shown). This photodiode can beintegrated in the active matrix display during processing. In this way,the degradation state of the dummy pixels can either be directlymeasured (light) or derived (from the relationship between voltageincrease and light decrease, as shown in FIG. 1).

In operation, the dummy pixels 11 can be used in several modes. In orderto take into account the shelf life effect, one or more of the dummypixels 11 remain essentially undriven, being only periodically probed byfurther monitoring unit 13 to establish the degradation state of thedummy pixel 11. As the probe period is short, this should not influencethe shelf life type of degradation. If degradation due to shelf life isdetected, the degradation state data must be taken into account byadjusting the pixel history in module 5 in an appropriate manner (i.e.by over-ageing all the display pixels 2) and thus adjusting the drivingsignal 8 to maintain the relative brightness level of the display pixels2.

In order to monitor that the pixel degradation is proceeding as expectedfrom the degradation model presented above, one or more of the dummypixels 11 (of each color) may be driven by the unit 13 (not shown).Preferably, these dummy pixels 11 may be driven so as to obtain anaverage brightness level of each colored sub-pixel on the display 1. Themonitored degradation state data can be used to adjust the pixel historyin the module 5 if strong deviations from expected behavior are foundand the controller 3 may generate an adjusted driving signal. This maymake it possible to also compensate for degradation during the “initialdrop” period, where degradation is less predictable. This could be animportant advantage, as it could eliminate the need to pre-age thedisplays prior to customer delivery, thereby increasing lifetime andreducing manufacturing time and costs.

In extreme situations, where serious deviations from the expecteddegradation behavior are encountered (e.g. degradation proceeds muchmore slowly than expected), means (not shown) are provided making itpossible to turn off the module 5 and thereby the compensation for oneor more colored display pixels 2. This will avoid any run-away behaviorby extreme overcompensation, which could lead to unnecessary earlyfailure of the display.

Next, an alternative embodiment is discussed with reference to FIGS. 4-7for improving the lifetime of an organic electroluminescent device. InFIG. 4 a schematic representation is shown of the brightness B decay fortwo types T1 and T2 of polymers used for a display 1 or a display pixel2 as a function of the fractional lifetime FL. Fractional lifetime isdefined as the time of operation divided by the lifetime for thatparticular device, wherein the lifetime is defined by the standardlifetime definition as the time in which the light output of the display1 or display pixel 2 decays by 50% compared to the initial value. Formatrix display applications a decay of only 10% may be allowed. So,especially for the type T1 behavior of the display 1 or display pixel 2,adjustment of the driving signal is important. Type I behavior isobserved for PPV-type conjugated polymers, having phenyl rings and vinylbonds, whereas the type T2 behavior is found for fluorine-typeconjugated polymers having only phenyl rings. It can be observed(indicated by the dotted lines in FIG. 4) that according to the 10%decay definition for matrix display applications the life time ofdisplay pixels using type T1 polymers is five to ten times less than fortype T2 polymers. According to the standard lifetime definition(allowing a decay of 50%) this difference would be only 10%. Especiallytype T1 polymers employed in display devices introduce severe problemswith regard to a uniform brightness over the display 1. These problemsrelate to the fact that a variation in brightness between display pixels2 is obtained if these pixels are driven for different amounts of time.Above, various embodiments have been described that account for thisbehavior and maintain the relative brightness of the display pixels 2,using a memory in module 5.

In the alternative embodiment of the invention shown in FIG. 5, thememory in the module 5 as described above with regard to the pixelhistory is not required to restore the original brightness level. Again,a display 1 comprises a plurality of display pixels 2 arranged in amatrix of rows and columns. The display pixels 2 can be driven by acontroller 3 in response to a data input signal 4. The data input signal4 comprises e.g. one or more images to be shown on the display 1 bydriving the individual display pixels 2. It will be appreciated that thedisplay 1 may be a passive or an active matrix display and may be eithera monochrome display or a color display in which the display pixelscomprise sub-pixels, for example, R, G and B. Circuitry 14 is providedfor applying a reverse current or reverse voltage to one or more of thedisplay pixels 2 and for measuring a resulting voltage or leakagecurrent. Connection 15 allows transmission of the signals required.Circuitry 14 further is adapted to deduce from the measurement results,the degradation state data of the display pixel 2. The degradation statedata thus obtained are input to the controller 3 via connection 16,enabling controller 3 to generate a driving signal 8 for the displaypixel 2 taking account of the degradation state data. It will beappreciated that circuitry 14 may be a module of e.g. the controller 3instead of being a separate entity.

FIG. 6 schematically shows the typical shift of the leakage currentI_(L) during lifetime t_(life), as indicated by the arrow, if a reversevoltage is applied. The time is indicated in terms of the lifetime.Measurements have been performed here under accelerated degradationconditions (90° C.; lifetime of 168 hours at 50 Cd/m²). At roomtemperature the corresponding lifetime amounts to approximately 22000hours. From FIG. 6, it is clear that by applying a reverse voltage V andmeasuring a leakage current I_(L), or vice versa, the time t duringwhich a display pixel 2 is driven can be determined.

FIG. 7 shows such a result for display pixels 2 if a reverse currentI_(L) is applied and a reverse voltage V is measured and linked to thefractional lifetime FL. The different symbols constitute the shift ofthe reverse voltage at three different reverse current densities. Alinear behavior is found for the voltage shift as shown by the line inFIG. 7 (deviation from this linear behavior is a precursor for failureof the display pixel 2). The measured voltage V is normalized to theinitial value V₀ for the reverse voltage. Since the devices can be madein a reproducible way, this initial value V₀ of the reverse voltage V isa constant.

In the alternative embodiment shown in FIG. 5, a specific reversecurrent I_(L) is applied by the circuitry 14 to preferably each displaypixel 2 and the voltage V is measured. The applied reverse current I_(L)suitable for performing this function scales with the size of thedisplay pixels 2. The application of the reverse current I_(L) to thedisplay pixels 2 may be executed e.g. once a day when turning on thedisplay device. As a result, for each pixel a reverse voltage V isobtained, which reverse voltage can be directly linked to the time t thedisplay pixels 2 have operated (see FIG. 7). This time is directlylinked to a brightness B, using the behavior of the display pixels 2shown in FIG. 4, which corresponds to a degradation state from which theadjustment on the data input signal 4 can be deduced in order tomaintain the relative brightness level of the display pixels 2 or torestore the original brightness level. So, the adjustment of the drivingsignal 8 can be done on the basis of the functional dependence betweenthe measured reverse voltage V and the corresponding required correctionof the display input signal 4, which is the same for all display pixels2. A memory for the pixel history is not required. For display pixels 2that show a large variation in properties or quality, a memory may beneeded for the initial voltage V₀. Data relating to this degradationstate are transmitted to the controller 3 via connection 16. Thecontroller 3 may generate a driving signal 8, taking account of thedegradation state data thus obtained, as a result of which the originalbrightness is corrected, restored or maintained at least partially.

In FIG. 8 an embodiment of the temperature sensor is shown wherein thedisplay device comprises an active display area, hereinafter referred toas display, with display pixels 2 arranged in a matrix of rows andcolumns. A possible configuration as used in PLED displays is that of adisplay pixel 2 or segment comprising a layer of electroluminescentmaterial with an active layer of organic material, which layer ispresent between a first and a second pattern of electrodes (not shown),which patterns define the display pixel 2 or segment, at least one ofthe two patterns being transparent to light to be emitted through theactive layer, and a first pattern comprising a material which issuitable for injecting charge carriers. The invention is also applicableto segmented displays, backlights, light sources and other lightemitting devices using PLED or OLED technology.

Moreover, the display device comprises an area 1 ¹ with reference pixels9 ¹¹. Since the reference pixels 9 ¹¹ are integrated into the displaydevice itself, more accurate sensing of the temperature of the actualdisplay pixels 2 can be achieved. In FIG. 8 the reference pixels 9 ¹¹have been implemented as separate pixels in the vicinity of the display1. However, it should be appreciated that also specific pixels of thedisplay 1 can be employed, e.g. the display pixels 2′ in the corners ofthe display 1.

The reference pixels 9 ¹¹ are preferably of a similar materialcomposition as the display pixels 2. This may depend e.g. on themanufacturing process employed for depositing the active layer. Ifspin-coating is applied, the material composition of the display pixels2 and the reference pixels 9 ¹¹ is similar. If inkjet printing isapplied, the material should be suitable for printing, but is notnecessarily similar for the materials employed for the display pixels 2and the reference pixels 9 ¹¹.

The display pixels 2 can be driven via connections 8 by a displaycontroller 3 in response to a data input signal 4.

In order to monitor the temperature of the display or display pixels 2,a temperature sensor controller 9 ¹ is employed. The temperature sensorcontroller 9 ¹ is connected to the reference pixels 9 ¹¹ via connections20 and to the display controller via connection 10. It will beappreciated that the temperature sensor controller 9 ¹ may be a moduleof the display controller 3 or other hardware, instead of being aseparate unit. The temperature sensor controller 9 ¹ may be applied forbiasing the reference pixels 9 ¹¹ as well as for measuring or deriving atemperature-dependent characteristic or value of the reference pixels 9¹¹.

The temperature of the display 1 or display pixels 2 is determined bythe temperature sensor controller 9 ¹. Temperature sensor controller 9 ¹measures a temperature-dependent characteristic or value of at least onereference pixel 9 ¹¹ or 2 ¹. Such a temperature-dependent characteristicor value may relate to electrical data of the reference pixel 9 ¹¹, suchas a current-voltage characteristic. These characteristics are obtainedby biasing the reference pixels 9 ¹¹. A bias current or voltage isapplied to the reference pixel 9 ¹¹ and a resulting voltage or currentis measured or derived. In FIG. 9 a schematic representation is shown ofa current I versus voltage V characteristic of a reference pixel 9 ¹¹.It is observed that for a temperature T, a current-voltagecharacteristic A is obtained that is different from the characteristic Bobserved at a temperature T₂, wherein in this situation T₂>T₁.Typically, temperatures range from 0 to 80° C. Voltages typically rangefrom −5 to 5 Volt in FIG. 9. The curves may vary in position and formdepending on e.g. differences of the supply lines of the referencepixels 9 ¹¹. The reference pixels 9 ¹¹ are not controlled by the displaycontroller 3 as they are not meant for display purposes. In fact it isbeneficial that the reference pixels 9 ¹¹ are biased in a temperaturemeasurement state by the temperature sensor controller 9 ¹. In thetemperature measurement state the reference pixel 9 ¹¹ is biased at alevel low enough to prevent, or at least substantially prevent, thereference pixel 9 ¹¹ from emitting light and high enough to enable areliable measurement or derivation of the temperature-dependentcharacteristic or value of the reference pixel as shown in FIG. 9. Thetemperature sensor controller 9 ¹ may comprise a unit for converting themeasured or derived temperature-dependent characteristic or value intothe (operating) temperature of the display pixels 2. Such a unit may bea look-up table wherein the obtained characteristic or value is linkedto a temperature. For example, a measurement or derivation of theconductivity of the reference pixels 9 ¹¹ by the temperature sensorcontroller 9 ¹, resulting in the characteristic A as shown in FIG. 9,can be linked to temperature T₁. The values in the look-up table mayhave been calibrated for disturbing effects such as electrical losses inthe connections 20 with the reference pixels 9 ¹¹ or built-in potentialsas a result of the materials applied. An other unit can be used as wellsuch as an analytical function lining a measured or derivedtemperature-dependent value of a reference pixel 9 ¹¹ to a temperatureof display pixels 2.

The temperature obtained by the temperature sensor controller 9 ¹ istransmitted to the display controller 3 via connection 10.

The display device shown in FIG. 8 comprises multiple reference pixels 9¹¹. These reference pixels 9 ¹¹ are preferably distributed so as to copewith temperature gradients over the display device.

Moreover, for color displays a reference pixel 9 ¹¹ may be employed forat least some of the colors R, G or B employed. This may increase theaccuracy of the temperature measurement. The temperature sensorcontroller 9 ¹ may need to have an appropriate look-up table to convertthe data of the separate reference pixels 9 ¹¹ into the righttemperature.

The reference pixels 9 ¹¹ are preferably not integrated in the activedisplay area. Instead it may be beneficial to shield the referencepixels 9 ¹¹ in the area 1 of the display device in order to avoidexposure of these reference pixels 9 ¹¹ to ambient or environmentallight. By shielding the reference pixels 9 ¹¹ photocurrents can beprevented as well as degradation due to ambient light, improving theaccuracy of the temperature measurement or derivation.

The temperature of the reference pixels 9 ¹¹ may be measured or derivedby the temperature sensor controller 9 ¹ continuously or probed only atspecific or periodic times or time intervals. Probing at specific timesinstead of measuring continuously may be advantageous with regard to thepower consumption of the display device. The time intervals for probingmay depend on e.g. the correction driving scheme employed. Moreover, ifthe light emitting layers of the LED are chosen such that the lightefficiency does not vary in a predetermined temperature range, thereference pixels 9 ¹¹ only have to be probed if the burn-in correctionhas to be determined.

For the purpose of teaching the invention, preferred embodiments of thedisplay device and the electronic device comprising such a displaydevice have been described above.

It should be noted that the above-mentioned embodiments illustraterather than limit the invention, and that those skilled in the art willbe able to design many alternative embodiments without departing fromthe scope of the appended claims. In the claims, any reference signsplaced between parentheses shall not be construed as limiting the claim.The word “comprising” does not exclude the presence of elements or stepsother than those listed in a claim. The word “a” or “an” preceding anelement does not exclude the presence of a plurality of such elements.The invention can be implemented by means of hardware comprising severaldistinct elements, and by means of a suitably programmed computer. Inthe device claim enumerating several means, several of these means canbe embodied by one and the same item of hardware. The mere fact thatcertain measures are recited in mutually different dependent claims doesnot indicate that a combination of these measures cannot be used toadvantage.

1. Display device for displaying an image comprising a plurality ofdisplay pixels (2), the display device comprising: sensors (9; 11; 14)for monitoring operating conditions of the display pixels (2), and acontroller (3) coupled to receive data related to the operatingconditions from the sensors (9; 11; 14) for determining a brightnesschange of the pixels (2) caused by the operating conditions, to generatea driving signal (8) for driving the pixels (2) in dependence on thebrightness change.
 2. Display device according to claim 1, wherein saidsensors (9; 11; 14) comprise at least one temperature sensor (9) formonitoring temperature data relating to the pixels (2), monitoring means(5) are present for monitoring total charge data of the pixels (2), andsaid controller (3) is adapted to generate said driving signal (8) independence on the total charge data and the temperature data.
 3. Displaydevice according to claim 2, wherein the controller is adapted to derivean acceleration factor from the temperature data and to adjust thedriving signal (8) depending on the product of the total charge data andthe acceleration factor.
 4. Display device according to claim 2, whereinthe temperature sensor (9) comprises at least one reference pixel andtemperature determination means adapted to determine a temperature independence on at least one temperature-dependent characteristic of thereference pixel.
 5. Display device according to claim 1, wherein thesensors (9; 11; 14) comprise at least one reference pixel (11),monitoring means (5) are present for monitoring total charge data of thepixels (2), and further monitoring means (13) are present, adapted fordetermining degradation state data of said reference pixel (11), saidcontroller (3) being adapted to generate said driving signal (8) takingaccount of said total charge data and said degradation state data 6.Display device according to claim 5, wherein a photodiode is present tomeasure the degradation state data of said reference pixel (11). 7.Display device according to claim 5, wherein the pixels (2) comprise atleast two sub-pixels of a different type, and at least one referencepixel for each type is present.
 8. Display device according to claim 5,wherein said controller (3) is adapted to provide each reference pixel(11) with a driving signal corresponding to an average brightness levelof the respective types.
 9. Display device according to claim 5, whereinsaid controller (3) is adapted to ignore at least one of the totalcharge data and the data from the sensors (9; 11; 14) for at least onesub-pixel.
 10. Display device according to claim 1, wherein the sensors(9; 11; 14) comprise means (14) to sense a relation between a reversecurrent and a reverse voltage of the pixels (2) for deriving degradationstate data for the pixels (2), and said controller (3) is adapted togenerate said driving signal (8) taking account of said degradationstate data.
 11. Display device according to claim 10, wherein said means(14) are adapted to derive said degradation state data when the displaydevice (1) is turned on.
 12. Method of generating a driving signal (8)for driving a plurality of pixels (2) of an organic electroluminescentdisplay device for displaying an image, the device comprising sensors(9; 11; 14) for monitoring operating conditions of the pixels (2); themethod comprising the steps of: obtaining data from the sensors (9;11;14) related to the operating conditions; determining a brightnesschange of the pixels (2) caused by the operating conditions; andgenerating a driving signal (8) in dependence on the brightness change.